The foundation for a factory-built house is the first concrete pour on the plot and the first place where such a house differs from one built on site: the elements do not give. If the foundation is two centimetres off or not level, the walls do not fit, and correcting that on site is expensive. That is why we talk about the house foundation so much before the first element goes into production.

Below are six things that decide which foundation a CLT house needs, what shapes its cost and how to write that line into your budget.

Why the foundation is special for a factory-built house

The walls of a house built on site are made on top of the foundation and adapted to it: if it is slightly higher or wider, the masonry or the frame adapts. Walls, floors and roof structure are instead cut to size in the factory weeks before erection and brought to the plot finished, so the foundation for a factory-built house has to match the design in millimetres, not centimetres.

So it is set out geodetically before the pour and surveyed after it: dimensions, diagonals, levels and flatness, with permitted deviations in millimetres, and the result is documented in a covered-works certificate with photos that later go into the occupancy permit file. Only then does our project manager release the production of the elements, which runs in parallel with the curing of the concrete. The shell is erected in about a week and a turnkey house is completed in about 22 weeks; the stages are described in our article on house construction stages.

That check is why we build the foundation ourselves for a turnkey house. With the house package the client can build it, but we hand over the precision requirements and survey the result before production.

1. Soil and bearing capacity

The first and biggest factor is the soil. Sand and gravel bear well and water passes through them; clay bears but swells and shrinks as moisture changes; peat and fill do not bear and require soil replacement or piles. The answer comes from a geotechnical survey, which we order before design when there is no data on the plot; a first overview of soil and groundwater is also available on the Land Board geoportal.

A survey is cheap compared with what unexpected soil costs during construction, and it is one of the plot criteria we wrote about in our article on buying a plot for a factory-built house.

2. Groundwater and drainage

The second factor is water. High groundwater or ground where water collects requires drainage, waterproofing and sometimes a higher plinth. For a timber house a dry perimeter is a matter of lifespan: the wall starts on top of the concrete and must stay dry.

In our solution, drainage and a damp-proof layer between the wall and the concrete are standard, and we build the drainage and the site routes ourselves as separate lines in the offer. We explained the effect of moisture on a timber structure in our article on timber house lifespan.

3. Frost depth and insulation

The third factor is cold. Estonian ground freezes deep in winter, and frost heave can move a house foundation that is not founded below the frost depth. The modern answer is insulation under the slab and on the sides, which keeps the ground beneath unfrozen and cuts the heat loss of the house.

Insulation is part of the energy class too: the junction of foundation and wall is a common place of heat loss, and in our design it is solved as a detail. What the energy class means is explained in our article on the energy class A house.

4. The mass and structure of the house

The fourth factor is the house itself. Cross-laminated timber walls weigh a fraction of masonry, so the loads are smaller and the foundation for a factory-built house can be lighter where the soil allows. At the same time a CLT house is a rigid box that carries those loads evenly downwards and needs a base that is level and flat.

We explained the load-bearing structure in our article on cross-laminated timber (CLT). The engineer sizes the foundation for the loads of the house and the bearing capacity of the soil, and that calculation is part of the building design the municipality checks in the building permit procedure under the Building Code.

5. Water underfloor heating and the slab

The fifth factor is the floor. Our houses have water underfloor heating, which affects the foundation type: the pipes are cast into the slab, under which there is insulation so the heat goes up into the room, not down into the ground. An insulated slab is therefore the most common foundation for a factory-built house: foundation, floor and heating carrier in one. How the pipe spacing, the heat source and the floor covering work together is the subject of our article on underfloor heating in a new house.

Flatness matters for the finishes as well: parquet and tiles need a flat base, and the more precise the pour, the less levelling later.

6. Site works and access

The sixth factor is everything around the pour: excavation, soil replacement, filling and compaction, routes under and next to the structure, the access road for the concrete mixer and later for the crane. These works depend on the plot more than on the house, and they are the reason the cost is not known before the plot has been inspected.

In our offer this line is stated with its assumptions and refined after the geotechnical survey. It is also why two houses of the same size end up with different square metre prices, a comparison trap we described in our article on why the price per square metre misleads. What a turnkey offer covers is set out in our article on the turnkey house price.

Foundation types in brief

The type follows from the interaction of these six factors. An insulated concrete slab foundation is the most common: fast, uniform, suited to underfloor heating. A strip foundation with an insulated floor suits a higher plinth or a sloping plot, and piles come into play on weak ground, where the load is carried down to the bearing layer. None is ruled out as a foundation for a factory-built house as long as the precision requirements are met.

Radon and other site-specific requirements

In some parts of Estonia the ground contains radon, against which a barrier is laid under the slab and the sub-slab ventilation solved. Whether the plot lies in a radon-risk area is shown by environmental data the architect checks, and the plan may set the height of the plinth, for example in a flood-risk area. These requirements go into the design before the pour.

What to send us

Send us the plot documents and, if possible, the soil data through the contact page, and the architect and the engineer will tell you which foundation for a factory-built house your plot calls for and whether a survey should be ordered before the quote. After a consultation and a look at your wishes we put together a more precise offer.